I recommend the EVGA 500W or 430W. They're basically as cheap as you can get while still being quality units. IMPORTANT EDIT: However, your unit can't really be upgraded by much, as the external power brick model is not friendly to standard power supplies.

You can't get much out of upgrading that prebuilt. Start anew, ideally once Zen announcements are formalized and you can get a wider selection of systems to build.

You're lucky, then. I'm not aware of uATX power supplies being a thing and it looks like standard ATX to me. The EVGA 500W is recommended, but at the same time, that computer has its days numbered due to the socket being dead.

Also, don't get the 4GB model. Performance will bottleneck you before VRAM will.